1. Field of the Invention
The present invention relates to a fuel cell stack comprising a stack body formed by stacking a plurality of power generation cells in a stacking direction. Each of the power generation cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. A coolant flow field is formed at least at one of positions between the power generation cells for allowing a coolant to flow in a direction along a power generation surface.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In general, a predetermined number of a plurality of power generation cells are stacked together in a stacking direction. At opposite ends of the power generation cells in the stacking direction, terminal plates are provided. Insulating plates are provided outside the terminal plates, and end plates are provided outside the insulating plates to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In some of power generation cells of the fuel cell stack, in comparison with the other power generation cells, the temperature is decreased easily due to heat radiation to the outside. For example, in the power generation cells provided at ends of the fuel cell stack in the stacking direction (hereinafter also referred to as the “end power generation cells”), since the heat is radiated to the outside from the terminal plates (current collecting plates) for collecting electrical charges generated in each of the power generation cells as electricity, or from the end plates for tightening the stacked power generation cells, the decrease in the temperature is significant.
Therefore, due to the decrease in the temperature, in the end power generation cells, in comparison with power generation cells in the central position of the fuel cell stack, water condensation occurs easily, and the water produced in the power generation cannot be discharged smoothly. In particular, when operation of the fuel cell stack is started below the freezing temperature, the water produced in the power generation by the end power generation cells may freeze undesirably. Thus, it is not possible to effectively raise the temperature in the end power generation cell. Consequently, the voltage of the fuel cell stack is low.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 8-130028 discloses a polymer electrolyte fuel cell as shown in FIG. 10. The polymer electrolyte fuel cell includes an end power generation cell 1. The end power generation cell 1 includes a membrane electrode assembly 2 and separators 3 and 4 sandwiching the membrane electrode assembly 2. The membrane electrode assembly 2 includes a fuel electrode (anode) 2b, an air electrode (cathode) 2c, and a polymer electrolyte membrane 2a interposed between the fuel electrode 2b and the air electrode 2c. The separator 3 has fuel gas grooves 3a on a surface facing the fuel electrode 2b, and has coolant grooves 3b on the opposite surface.
The separator 4 of the end power generation cell 1 has oxygen-containing gas grooves 4a on a surface facing the air electrode 2c. No coolant grooves are formed on the opposite surface of the separator 4. According to the disclosure of Japanese Laid-Open Patent Publication No. 8-130028, in the structure, the separator 4 is not cooled excessively by the coolant. That is, the end power generation cell 1 is not cooled excessively.
However, in the conventional technique, since the coolant does not flow outside the end power generation cell 1, the end power generation cell 1 may not be cooled sufficiently. Though not shown, a terminal plate, an insulating plate, and an end plate (tightening plate) are stacked on the end power generation cell 1. Since the insulating plate is made of resin and heat conductivity of the insulating plate is low, heat radiation amount is limited. Therefore, in comparison with the power generation cells at the central position, the end power generation cell 1 has a considerably high temperature. Components such as the polymer electrolyte membrane 2a are degraded easily, and the durability of the end power generation cell 1 is poor.